Ball-point pen ink compositions which have conventionally been used comprise a dye or pigment as colorant, a resin (a ketone resin or xylene resin), a high-boiling solvent (phenyl Cellosolve, phenyl glycol, benzyl alcohol or the like) and various additives. When letters or lines are written on paper with such conventional ink, the ink permeates even in the interiors of fibers of paper and the ink dyes the fibers or is adsorbed on the fibers. Accordingly, written letters or lines cannot be erased by an ordinary rubber eraser. As means for erasing such ball-point pen ink traces, there have been adopted a chemical decomposition method in which the colorant is decomposed by oxidation and a physical method in which traces are rubbed off by a sand-incorporated rubber eraser. The chemical decomposition method, however, is defective in that the majority of colors, inclusive of colors of inks comprising a pigment as the colorant, cannot be erased and, even when certain erasable colors are employed, yellowing is caused after removal of the colors. Furthermore, the physical method is defective in that although erasing is possible, the paper surface is rubbed off at the erased part and in case of thin paper, breakage of the paper is readily caused by the erasing operation. Accordingly, no satisfactory erasing effect can be attained by any of these erasing methods.
Recently, a ball-point pen ink erasable by an ordinary rubber eraser has been developed. For example, a ball-point pen ink of this erasable type comprising a natural rubber, a low-boiling solvent, a high-boiling solvent and pigment particles as colorant is known (U.S. Pat. No. 4,097,290). Although traces of such ball-point pen ink can be erased by a rubber eraser within several hours from the time of writing, these traces, like traces of conventional ball-point pen inks, cannot be erased by a rubber eraser after passage of a few days. Furthermore, this ball-point pen ink is defective in that the speed of drying of the ink just after writing is very low and the ink is left sticky. Therefore, if other paper is placed on the written surface, both the papers adhere to each other. Moreover, since the applied ink just after writing is readily peeled from the paper surface, if traces are touched by a hand or finger just after writing, the paper surface or the hand or finger becomes dirty with the ink.